[Bug middle-end/18041] New: OR of two single-bit bitfields is inefficient

[kazu at cs dot umass dot edu]

Consider:

struct B {
  unsigned bit0 : 1;
  unsigned bit1 : 1;
};

void
foo (struct B *b)
{
  b->bit0 = b->bit0 | b->bit1;
}

./cc1 -O2 -fomit-frame-pointer -mregparm=3 generates

foo:
        movb    (%eax), %dl  <- one load
        movb    %dl, %cl
        shrb    %cl
        orl     %edx, %ecx
        andl    $1, %ecx
        movl    (%eax), %edx <- another load from the same place
        andl    $-2, %edx
        orl     %ecx, %edx   <- the second OR
        movl    %edx, (%eax)
        ret

We could do something like

        movb    (%eax), %cl
        movb    %cl, %dl
        shrb    %dl
        andl    $1, %edx
        orl     %ecx, %edx
        movb    %dl, (%eax)
        ret

or

        movb    (%eax), %dl
        testb   $2, %dl
        je      .L6
        orl     $1, %edx
        movb    %dl, (%eax)
.L6:
        ret

expr.c actually has code intended to emit the second suggestion
(look for "Check for |= or &= of a bitfield" in expr.c), but it is
practically disabled because we get tree like this

  b->bit0 = (<unnamed type>) (unsigned char)
              ((signed char) b->bit0 | (signed char) b->bit1)

whereas the code in expr.c expects

  b->bit0 = b->bit0 | b->bit1;

The code is not triggered even in gcc-3.3.  Probably it is practically
disabled for a long time.

-- 
           Summary: OR of two single-bit bitfields is inefficient
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P2
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18041